This invention relates to the decoding of prerecorded servo track positioning information, and, more particularly, to the decoding of servo track positioning information from adjoining servo tracks having different servo patterns as read by a servo head to allow positioning of the servo head and of read/write elements which are at an indexed position with respect to the servo head.
In the data storage industry, advances in technology include increases in the data storage capacity of given data storage media. One means of increasing the data storage capacity of data storage media, such as magnetic tape cartridges or magnetic tape cassettes, is to increase the track density of the data storage media, and in a corresponding manner, to decrease the width of each track.
In a typical magnetic tape, data is recorded in a plurality of parallel, longitudinal data tracks. A data head may have a plurality of data heads which have fewer numbers of read/write elements than tracks. The data tracks are divided into groups, typically interleaved, and the data head is indexed laterally with respect to the tracks to access each group of data tracks. In order to properly register the data head with the data tracks, prerecorded servo tracks are provided which are parallel to the data tracks. A servo read head located at an indexed position with respect to the read/write elements reads the servo tracks. The servo tracks provide lateral positioning information which, when read by the servo read head, can be decoded to indicate whether the servo read head is correctly positioned with respect to the servo tracks. Thus, the servo head can be moved laterally to a desired position with respect to the servo tracks so as to properly register the read/write elements with respect to a desired group of data tracks. Then, the servo head can follow the servo tracks as the media and the head are moved longitudinally with respect to each other, so that the read/write elements maintain registration with the data tracks.
As an example, the prerecorded servo track positioning information comprises adjoining servo tracks having different servo patterns, one of the servo patterns comprising a constant amplitude signal of a single first frequency, and the other servo pattern alternating between a constant amplitude burst signal of a single second frequency and a zero amplitude null signal. The resultant signal read by the servo head is a maximum signal comprising the first frequency signal combined with the second frequency burst signal and a minimum signal comprising the first frequency signal combined with the null signal. If the servo head is correctly positioned at the junction of the adjoining servo tracks, the amplitude of the combined first and second frequency signals is twice the amplitude of the combined first and null signals, and is easily decoded. Coassigned U.S. Pat. No. 5,448,430 illustrates the above discussed servo track patterns and describes a track following servo positioning system employing peak detection to determine the maximum and minimum signals.
As data capacity is increased, it is also desirable to have backward compatibility to data storage media having the prior level of data capacity, to avoid the necessity of copying all of the data recorded on the prior media onto the new media.
As the result, it is desirable to increase the track density of a data storage media, while utilizing the prior media servo tracks, so that the servo system may be operated to utilize the servo track positioning information as before for the prior media, and to utilize the same servo track positioning information in a more precise manner to access tracks at a higher track density. Thus, it becomes necessary to accurately measure the maximum and the minimum signals at servo head positions not at the center directly between the adjoining servo tracks. The problem becomes especially difficult as the servo head is positioned more directly over the first single frequency track, in that the minimum signal becomes closer to the maximum signal. It becomes difficult to differentiate between the two signals, difficult to accurately measure each of the two signals, and therefore difficult to determine the ratio between the two signals. Thus, the precise positioning of the servo head and the corresponding read/write elements also becomes difficult.
An object of the present invention is to provide decoding of servo track positioning information from adjoining servo tracks having different servo patterns to provide a broad dynamic range of positioning information.
Disclosed are a servo track decoder and method for decoding asynchronous digital samples of prerecorded servo track positioning information. The prerecorded servo track positioning information comprises adjoining servo tracks having different servo patterns, one of the servo patterns comprising a maximum a constant amplitude signal of a single first frequency, and the other servo pattern alternating between a constant amplitude burst signal of a single second frequency and a zero amplitude null signal. The resultant signal read by the servo head is a maximum signal comprising the first frequency signal combined with the second frequency burst signal and a minimum signal comprising the first frequency signal combined with the null signal. A digital servo detector asynchronously samples the signals read by the servo head.
An envelope follower receives the asynchronous digital samples, detecting and providing a maximum envelope output measuring the amplitude of a burst envelope of the maximum of the asynchronous digital samples, and detecting and providing a minimum envelope output measuring the amplitude of a burst envelope of the minimum of the asynchronous digital samples.
A xe2x80x9cDROPOUTxe2x80x9d threshold detector receives the asynchronous digital samples and detects the received asynchronous digital samples failing to meet a xe2x80x9cDROPOUTxe2x80x9d threshold related to the maximum burst envelope, providing a xe2x80x9cDROPOUTxe2x80x9d threshold detection signal. An xe2x80x9cACQUIRExe2x80x9d detector is coupled to the xe2x80x9cDROPOUTxe2x80x9d threshold detector and responds to the xe2x80x9cDROPOUTxe2x80x9d threshold detection to detect the minimum envelope for the envelope detector, which provides the minimum envelope output. The xe2x80x9cDROPOUTxe2x80x9d detection distinguishes the minimum envelope from the maximum envelope and allows measurement of the minimum envelope, whereby a ratio of the measured maximum envelope amplitude output and the measured minimum envelope amplitude output represents the lateral position of the servo head.
In further embodiments of the invention, in xe2x80x9cTRACKINGxe2x80x9d mode, the digital samples may be qualified as exceeding the xe2x80x9cDROPOUTxe2x80x9d threshold to provide the maximum envelope. The xe2x80x9cDROPOUTxe2x80x9d threshold detector detects a predetermined programmable number related to the number of sequentially received digital samples that fail to meet the xe2x80x9cDROPOUTxe2x80x9d threshold.
The envelope detector may additionally employ an envelope filter for filtering, with a preceding envelope amplitude, an error amplitude between a qualified digital sample and the preceding envelope amplitude, the error amplitude multiplied by a provided programmable xe2x80x9cTRACK ATTACKxe2x80x9d gain value upon the error amplitude indicating an increase in the envelope amplitude, and the error amplitude multiplied by a provided programmable xe2x80x9cTRACK DECAYxe2x80x9d gain value upon the error amplitude indicating a decrease in the envelope amplitude. The envelope amplitude is amplified by a multiplier value, also called a multiplication xe2x80x9cfactorxe2x80x9d, in the xe2x80x9cDROPOUTxe2x80x9d threshold detector, by a xe2x80x9cDROPOUTxe2x80x9d factor to have the effect of reducing the xe2x80x9cDROPOUTxe2x80x9d threshold with respect to the envelope amplitude.
In a still further embodiment, upon the digital samples failing to meet the xe2x80x9cDROPOUTxe2x80x9d threshold, the xe2x80x9cDROPOUTxe2x80x9d threshold detector switches to an xe2x80x9cACQUIRE DECAYxe2x80x9d mode, and an error amplitude filter filters, with the preceding envelope amplitude, an error amplitude between any digital samples, even though they are not qualified, and the preceding envelope amplitude, the error amplitude multiplied by a provided predetermined xe2x80x9cACQUIRE DECAYxe2x80x9d gain. Additionally, the xe2x80x9cACQUIRExe2x80x9d detector detects a continuous sequence of qualified digital samples exceeding the amplitude threshold for a programmable predetermined number of digital samples. The xe2x80x9cACQUIRExe2x80x9d detector switches out of the xe2x80x9cACQUIRExe2x80x9d mode.
The envelope detector may continually update the respective measured envelope amplitude with the most recent filtered envelope amplitude. Additionally, the envelope detector may comprise latches which load the maximum and minimum output amplitudes, resetting the measured maximum output logic amplitude to zero, and resetting the measured minimum output logic amplitude to the greatest value.
Thus, even as the servo head is positioned more directly over the first single frequency track where the minimum signal becomes closer to the maximum signal, the present invention differentiates between the two signals, and measures the envelopes of each of the two signals, allowing determination of the ratio between the two signals. Thus, the present invention provides decoding of the servo track positioning information to provide a broad dynamic range of positioning information.
For a fuller understanding of the present invention, reference should be made to the following detailed description taken in conjunction with the accompanying drawings.